The present invention relates generally to calibration standards, and more particularly, to a standard used to calibrate an eddy current inspection probe.
Eddy current probes are commonly used to detect flaws in surfaces of manufactured parts such as gas turbine engine components. During this type of inspection, electromagnetic induction is used to induce eddy currents in the part being inspected. One or more drive coils inside the probe generate alternating magnetic fields which induce the eddy currents in the part when the probe is moved close to the part. When flaws are present in the part, the flow of eddy currents is altered. The altered eddy currents produce changes in a secondary magnetic field which are detected by the drive coil(s) or by separate sense coils inside the eddy current probe. These coils generate an electrical signal in response to the altered secondary magnetic field. The amplitude of electrical signal is generally proportionate to the size of the flaw within the range of flaw sizes for which the probe is designed. Thus, the size and location of flaws may be detected using eddy current probes.
One type of eddy current probe used for inspecting part features having complex shapes includes an array of drive coils and sensing coils for simultaneously inspecting various portions of the feature. By simultaneously inspecting all portions of the feature, the need to pass the probe over the feature more than once is eliminated. Because only one pass is required, this type of probe has the advantage of reducing inspection time.
Eddy current probes and the associated electronic devices must be calibrated to normalize the response of the coils in the array and to establish the system gain so the amplitude of the response can be correlated to particular flaw sizes. In the past, calibration standards having the specific part geometry have been used to normalize the response of the coils and a separate flat plate having a manufactured flaw of a known size has been used to independently establish system gain. However, this multi-step calibration process takes time. Attempts to make a single pass calibration standard have resulted in large variations in calibration. Because the eddy current inspection results are used to determine the length of service remaining in the inspected part, large variations in calibration result in underestimated part life prediction and premature part retirement which decrease acceptable service intervals and increase maintenance cost.
Briefly, apparatus of this invention is a calibration standard for calibrating an eddy current inspection probe sized and shaped to inspect a preselected non-planar feature of a manufactured part. The feature extends in a longitudinal direction and in a lateral direction. Further, the feature has an end profile as viewed in the longitudinal direction having a substantially invariant shape and orientation. The calibration standard includes a body having a non-planar surface extending in a longitudinal direction and in a lateral direction. The standard also has an end profile as viewed in the longitudinal direction of the surface substantially identical to the profile of the feature. The surface of the body has an elongate narrow opening extending into the body substantially normal to the surface and traversing the surface of the body at a substantially constant angle with respect to the longitudinal direction of the surface as viewed normal to the surface.
In another aspect, the invention includes a method of manufacturing a calibration standard including forming first and second pieces of a body so each of the pieces has a surface extending in a longitudinal direction and a lateral direction, so each piece has a profile in the longitudinal direction of the respective surface generally similar to the profile of the feature, and so the first and second pieces have mating faces. The method also includes positioning the face of the first piece of the body adjacent the face of the second piece of the body and aligning the surface of the first piece of the body with the surface of the second piece of the body. In addition, the method includes simultaneously machining the surfaces of the first and second pieces of the body after the faces are positioned adjacent each other and the surfaces are aligned to ensure the profiles in the longitudinal directions of the first and second pieces are substantially identical to the profile of the feature and to remove any discontinuities in the surfaces adjacent the faces thereby to eliminate variations in eddy current inspection probe signal during calibration resulting from discontinuities in the surfaces.
In yet another aspect of the present invention, the method includes forming first and second pieces of a body, applying a voltage potential to the first piece of the body relative to the second piece of the body. The method also includes submersing the first and second pieces in an oil and positioning the face of the first piece of the body adjacent the face of the second piece of the body. Further, the method includes moving at least one of the first and second pieces of the body toward the other of the pieces so at least portions of the faces contact each other to simultaneously machine the faces of the pieces by electrical discharge machining.
Other features of the present invention will be in part apparent and in part pointed out hereinafter.